Method of direct potentiometric analysis of a liquid sample

ABSTRACT

1. A METHOD OF POTENTIOMETRIC ANALYSIS OF A SERIES OF LIQUID SAMPLES FOR A CONSTITUENT OF INTEREST UTILIZING A REFERENCE ELECTRODE AND A SENSING ELECTRODE HAVING AN ION-SELECTIVE SURFACE PORTION, SAID REFERENCE ELECTRODE AND SAID ION-SELECTION SURFACE PORTION OF SAID SENSING ELECTRODE DEFINING, IN PART, A FLOW-THRU CONDUIT, COMPRISING THE STEPS OF: FLOWING THE LIQUID SAMPLES SEQUENTIALLY IN A STREAM ALONG SAID CONDUIT; INTRODUCING GAS TO SEGMENT THE STREAM OF SAMPLE TO FORM GAS SEGMENTS INTERMEDIATE EACH SAMPLE AND ITS NEIGHBORING SAMPLES SO THAT EACH SAMPLE COMPRISES A LIQUID SEGMENT; PASSING SAID FLOWING STREAM ALONG SAID CONDUIT TO CONTACT SAID REFERENCE ELECTRODE AND SAID ION-SELECTIVE SURFACE PORTION OF SAID SENSING ELECTRODE WITH SAID SAMPLE LIQUID AND GAS SEGMENTS IN SAID FLOWING STREAM; LOCATING SAID ELECTRODES WITH RESPECT TO EACH OTHER SUCH THAT EACH SAMPLE LIQUID SEGMENT IN SAID STREAM BRIDGES SAID REFERENCE ELECTRODE AND SAID ION-SELECTIVE SURFACE PORTION OF SAID SENSING ELECTRODE; AND MEASURING THE ELECTRICAL POTENTIAL ACROSS SAID ELECTRODES FOR ANALYSIS OF SAID LIQUID SAMPLE SEGMENTS.   D R A W I N G

T. M. Asr ErAL 3,840,438

METHOD OF DIRECT PONTENTIOIETRIC ANALYSIS OF A LIQUID SAMPLE Oct. 8, 1974 Filed April l0. 1972 kwkkbm W United States Patent O 3,840,438 METHOD OF DIRECT POTENTIOMETRIC ANALYSIS OF A LIQUID SAMPLE Therese M. Ast, Tarrytown, Milton H. Pelavin, Chappaqua, and K. Jagan Mohan Rao, Tarrytown, N.Y., assignors to Technicon Instruments Corporation, Tarrytown, N .Y.

Filed Apr. 10, 1972, Ser. No. 242,556 Int. Cl. G01n 27/46 U.S. Cl. 204-1 T 7 Claims ABSTRACT OF THE DISCLOSURE The method, susceptible of automation, of monitoring a stream or analyzing up to a very fast rate a series of samples by direct potentiometric measurements, particularly useful for sodium determinations, which samples are separated from one another by immiscible fluid segments in a conduit, which segments cleanse the conduit and maintain sample integrity, utilizing both an ionselective electrode and a reference electrode associated with the segmented stream. The avoidance of the removal of the immiscible uid segments of the stream prior to analysis enables a faster rate of analysis. The method includes the step in the analysis of blood samples of conditioning such a sodium ion electrode, immediately before running a first series of such samples, by exposure over a period of time to blood protein and the blood ion constituents which will affect the sodium ion electrode on exposure of the later to the samples, as well as buffer of the type used in analysis. Human blood serum or plasma is useful for such conditioning. Such conditioning effectively tends to avoid drift and speeds the response of the sodium ion electrode. To further speed the response of the sodium ion electrode for faster analysis of each sample, the samples are heated so as to have a temperature above 40 C. on exposure of the samples to the last-named electrode. Such fast electrode response reduces to an insignicant factor potassium transient effects in sodium analyses of blood samples.

BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to a method of measuring ionic parameters, especially those of sodium, potentiometrically or electrochemically in monitoring a stream or analyzing up to a very fast rate a series of samples.

2. Prior Art Heretofore automated methods have been employed for the quantitative wet-chemical analysis of a constituent in a liquid sample flowing in a monitoring stream or for a constituent of a series of different samples owing in a stream which samples are separated from one another by immiscible fluid segments such as gas or air bubbles. Commonly, though not always, the stream is debubbled before passing through an analysis station which may include a colorimeter. Apparatus for carrying out such an automated method is illustrated and described in Skeggs U.S. Pat. 2,797,149 issued June 25, 1957 and by Skeggs U.S. Pat. 2,879,141 issued Mar. 24, 1959. Such apparatus, wherein a sodium selective glass electrode was substituted for the colorimeter, has been used for sodium determinations in the analysis of urine on an automatic and continuous-now basis, as reported by Harold Jacobson in Analytical Chemistry, Vol. 38, No. 13, pp. 1951-1954, December 1966. The sodium ion glass electrode used was similar to that illustrated and described in Arthur et al. U.S. Pat. 3,398,079 issued Aug. 20, 1968, identified in the Jacobson article as Beckman sodium ion glass ice electrode No. 39046. In the last-mentioned reported sodium analysis, the segmented sample stream was debubbled prior to exposure of the sodium ion glass electrode to the sample stream. The sodium ion electrode was associated with a conventional reference electrode providing what is known as a leak junction in communication with the sample stream to complete an electrical circuit. The sodium ion electrode used was also of the bulb type having a portion of the bulb, which was exposed to the sample, constructed of sodium-sensitive glass. The analysis rate of samples was relatively very slow.

In an article titled lon-Selective Electrodes in Contnuons-Flow Analysis Determination of Calcium in Serum by I. Ruzicka and I. C. Tjell in Anal. Chim. Acta, 47 (1969) pp. 479-482, published in Amsterdam by Elsevier Publishing Co., there is illustrated and described a technique generally similar to that described by Jacobson, supra, differing principally in the particular electrodes used and in the use in the latter of a heating bath to heat a sample stream to 25 C. prior to exposure of the sample stream, after debubbling the stream, to the electrodes. The last-mentioned article describes a sample rate of analysis of 20-60 samples an hour with an interaction between samples increasing to 7% at the sampling rate of 60 samples per hour.

In the analysis of blood samples, for example, one factor which has slowed the rate of analysis of such samples has been the well known potassium transient effect on the sodium ion glass electrode which responds initially and confusingly to the presence in these samples of potassium, as documented, among others, by S. M. Freedman on pp. 457, 458 of Glass Electrodes for Hydrogen and Other Cations, G. Eisenman (Ed.) Marcel Dekker, Inc., New York, 1967, wherein such potassium transient response is also illustrated.

It is now commonly recognized that the application of heat to an ion-selective glass electrode, as in iuoride measurements, speeds up the response of the electrode to the sample. However, too much heat applied to the ionselective glass of such an electrode during its operation destroys its usefulness as an electrode, Iand the application of heat alone does not result in rates of analysis even approaching as fast a rate as 200 samples an hour.

It has been common practice to attempt to speed the response of an ion-selective glass electrode by conditioning it prior to use by exposure to a solution containing the ions to which it will be subjected in use. yIt was previously thought that conditioning a sodium ion glass electrode by exposure of the electrode to blood sera or plasma, used in blood analysis, would coat the sodium-sensitive glass with blood protein present in such sera or plasma and that this would result in slower electrode response and loss of sensitivity.

In the type of sodium determinations utilizing continuous ow described by Jacobson, supra, it is conjectured that exposure of the sodium-selective glass electrode to bubbles in the sample stream was avoided because of the known polarizing effect of relatively rapid repetitive exposure to air and liquid which not only causes erratic potentials but which is also damaging to the sodiumsensitive glass and may in time destroy it for electrode purposes. While the sodium-sensitive glass surface of the ion-selective electrode used by Jacobson constituted a surface portion of a glass bulb of the electrode as previously indicated, sodium-sensitive glass has been used heretofore in the shape of a cannula or capillary tube for receipt of a sample in an ion-selective electrode as disclosed by Schiller U.S. Pat. 3,357,910 issued Dec. 12, 1967. In the last-mentioned patent, the sodium-sensitive glass capillary is illustrated and described only with reference to a batch-type of anlysis as opposed to continuousflow type analysis.

making sodium measurements.

SUMMARY OF THE INVENTION One object of the invention is to provide an improved method of potentiometric analysis of various liquid samples, which offers the advantages of simplicity, ease of operation and precision which is as good as, if not better than, flame photometric methods.

Another object is to provide such a method, susceptible of automation, of monitoring a stream or analyzing up to a very fast rate a series of samples by direct potentiometric measurements, particularly useful for sodium determinations, which samples are separated from one another by immiscible fluid segments in a conduit, which segments cleanse the conduit and maintain sample integrity, utilizing both an ion-selective electrode and a reference electrode associated with the segmented stream, and wherein both the reference and the indicator electrodes are exposed to the segmented stream. Such analysis of discrete samples makes possible a much faster rate of analysis. Contrary to prior thinking, polarization of the electrodes is avoided. The method includes the step in the analysis of blood samples of conditioning such a sodium ion electrode, immediately before running a first series of such samples, by exposure over a period of time to blood protein and the blood ion constituents which will affect the sodium ion electrode on exposure of the latter to the samples, as well as the type of buffer solution used in analysis. Human blood serum or plasma is useful for such conditioning. Such conditioning, contrary to prior thinking, effectively tends to avoid drift and speeds the response of the sodium ion-selective electrode. The response of the sodium ion-selective electrode is further speeded for faster analysis of each sample by heating the samples so that they have a temperature above 40 C. on exposure of the samples to the last-named electrode.

Such fast electrode response reduces to an insignificant.

factor potassium transient effects in sodium analyses of blood samples.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a diagrammatic view illustrating a system useful for carrying out a method of analysis embodying the invention; and

FIG. 2 is a somewhat diagrammatic view illustrating in elevation a useful form of ion-selective electrode and reference electrode for analysis in accordance with FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of analysis both illustrated and described herein is with reference to sodium determinations. However, it is to be understood that the method of the invention is applicable to the determination of other substances such as calcium, for example, utilizing an appropriate ion selective electrode and a reference electrode. As illustrated in the system of FIG. 1, there is shown a sample conduit in which a monitoring stream of sample or a series of different samples such as human blood sera or plasma is caused to flow from a source, not shown, through an inlet 12 by a pressure differential for a sodium determination. 'Ihe sample stream may be supplied, as by a peristaltic pump, from a sampler such as illustrated and described in Skeggs U.S. Pat. 2,879,141, supra, with immiscible fluid segments separating the samples from one another. Such immiscible fluid segments may be formed of a gas and the stream may be further segmented by segments of a wash solution separated from the samples by segments of the immiscible fluid such as i1- lustrated and described by de Jong, `U.S. Pat. 3,134,263 issued May 26, 1964. In the illustrated form, sample may flow at a volumetric rate of between approximately 200 microliters per minute. The -ow from an outlet of conduit 10 enters an adjoining inlet of conduit 14.

To conduit 14 there is added through an outlet of a conduit 16 an appropriate buffer solution to dilute each sample and provide both pH and ionic strength adjustment. The buffer solution is caused to flow into the inlet 18 of the conduit 16 from a source, not shown, by a pressure differential, and a flow controller, as indicated, is interposed in the conduit 16 to control the ow of the buffer solution into the conduit 14, such controlled flow of the last-mentioned solution being approximately SOO-1000 microliters per minute.

In the form of the invention under discussion, the buffer solution flowing to the conduit 14 from the conduit 16 is segmented at a controlled rate by air under pressure owing in a conduit 20 through inlet 22 from a conventional source not shown. The thusly combined segmented sample and buffer streams received in the conduit 14 flow through a mixing coil interposed in conduit 14, wherein the sample segments and the buffer solution are intermixed. On passing from the mixing coil as shown in FIG. 1, the thusly treated sample stream is heated to a temperature above body temperature and approximately between 40-50" C. prior to exposure to an ion-selective electrode which in this case is a sodium ion-selective electrode. For this purpose a conventional heating bath may be employed as shown in FIG. 1.

Also as shown in the last-mentioned view, a three-way valve 24 is interposed intermediate the outlet of the mixing coil and the inlet of the heating bath. The valve 24 has an outlet connected to a waste bypass as shown so that when it is desired to flush out the aforementioned liquid conduits extending toward and through the mixing coil with a flushing solution, such flushing solution is diverted so that it does not reach and does not affect the sodium ion-selective electrode shown in FIG. 1 as being associated with the outlet of theA treated sample stream from the heating bath.

Downstream from the heating bath in conduit 26 in communication through valve 24 in one position of the latter with the outlet of conduit 14, there is provided a conductor pin, not shown, extending into the conduit 26 for exposure to the stream therein and connected to a ground wire 28, which pin and wire 28 are located upstream of the sodium ion electrode indicated in FIG. 1.

The choice of a sodium ion-selective electrode and a reference electrode structure is not critical to the invention, provided that the ion-selective electrode is of the continuous-flow type and the electrical connection of the reference electrode to the sample stream is close to the ion-sensitive surface of the ion-selective electrode. A very satisfactory combination electrode structure is illustrated diagrammatically in FIG. 2. This electrode structure is illustrated and described in the co-pending Brand and Rao U.S. Patent application assigned to the same assignee, Ser. No. 242,507 filed on Apr. 10, 1972. Hence the electrode structure, indicated generally at 30, does not require a detailed description here.

The electrode structure 30 comprises a body member 32 of a block-like form structured of an insulating material and having a bore 34 therethrough enlarged as at 36. In the bore 34 arranged axially thereof is a cannula 38 structured of sodium-selective glass of any known suitable type and fixed in the bore 34 by any suitable means.

As shown in FIG. 2, the cannula 38 has an outer diameter considerably less than the enlargement 36 of the bore and extends to the right beyond the block 32. The inner diameter of the cannula 38 is flush with the portion of the bore 34 of smaller diameter. The cannula 38 extends through a suitable seal 40, which seal forms with the body member 32 and the cannula 38 in the area of the bore enlargement 36 a chamber 41 for the electrolyte filling solution for the internal reference electrode. The chamber 41 is preferably provided with a filling port 42 in the body member 32 which may be closed by a suitable plug not shown. The cannula 38 in the area where it passes through the chamber 41 is completely surrounded by the electrolyte solution in the chamber. The ion-selective half cell is completed by an internal reference electrode 44 of conventional silversilver chloride wire which is shown extending into the body member 32 and into the electrolyte in the chamber 41. The inlet end 46 of the cannula 38 is suitably connected to the outlet of the conduit 26 of FIG. l.

The reference electrode portion of the combined electrode structure 30 includes a conduit 48 in the member 32 (FIG. 2) the inlet of which is connected to the outlet of a conduit 50 (FIG. l) having an inlet end connected to an electrolyte reservoir which electrolyte is here shown as potassium chloride (KCl). The reservoir is sealed and is pressurized through a conduit 52 having an outlet connected to the reservoir and having an inlet end 54 connected to a source of air under pressure, not shown. The KCl reservoir of FIG. 1 may be supplied with KCl solution by any suitable means not shown. The conduit 50 conveys KCl under pressure to conduit 48 into which conduit electrode 56 extends formed of siliver-silver chloride wire, for example, forming with the KCl solution the reference half cell.

The reference electrode portion of the combined electrode structure 30 is shown as being of the leak-junction type for electrical connection to the segmented, treated sample stream. As previously indicated, this stream enters the sample passageway 34 through the cannula 38 inserted in the inlet portion of the passageway-forming bore 34. The sample stream exits from the cannula 38 into the last-mentioned passageway which is continued through the axis of a boss 60 of the block member 32 which boss is formed as a truncated cone. The aforementioned conduit 48 for pressurized KCl extends through a face of the block member 32 so as to have an outlet in a location above and closely adjacent the base of the cone-like boss in a planar annular surface of the block member 32.

A block-like body member 62 coacts with the body member 32. The block-like member 62, structured of an insulating material, has a sample passageway 64 extending therethrough into the outlet end of which a nipple 66 is inserted for connection to the inlet end of a waste conduit 68 (FIG. 1) which has an outlet end connected to a suitable waste receptacle or drain not shown. The inlet end of the sample passageway 64 is through the bottom of a recess 70 having a shape complemental to the cone-like boss 60 of the member 32. Arranged circumferentially around the margin of the mouth of the recess 70 is a yieldable annular gasket 72 carried by one of the block members 32, 62 to provide a seal between the block members as shown in FIG. 2. As indicated in the last-mentioned View, the block members 32 and 62, provide a liquid passageway therebetween in communication with the outlet of the KCl conduit 48 and defined at least in part by the boss 60 and the recess 70.

This construction provides a leak junction between the KCl electrolyte of the reference electrode and the segmented sample stream flowing from the block 32 through the outlet thereof in the cone 60 to the inlet of the passageway 64 for the sample stream provided in the block 62. In actual practice the leak junction of the reference electrode may -be spaced only 2 mm. from the sodiumsensitive glass cannula 38 of the ion-selective electrode. While not shown here, the block-like members 32 and 62 may be supported for relative adjustment toward and away from each other for restriction or enlargement of the leak junction therebetween. Also, in actual practice, the cross-sectional size of the passageway for the leak junction between the block-like members 32, 62 is relatively small. Furthermore, only a very small volume of the KCl solution is permitted to enter the sample stream. It will be understood from the foregoing that the leak junction is formed entirely around the periphery of the sample stream.

As indicated in FIG. 1, the sodium-selective electrode is interconnected to an input terminal of a differential amplifier by a conducter 76. The conductor 76 is connected to the silver-silver chloride wire 44 of this electrode. The reference electrode is interconnected to a second input terminal of the differential amplifier by a conductor 78. The conductor 78 is connected to the silver-silver chloride wire 56 of the reference electrode. The aforementioned ground wire or conductor 28, connected to the aforementioned conductor pin (not shown) exposed to the stream in the conduit 26, interconnects this pin to a third input terminal of the differential amplilier which amplifier has an output terminal as shown which may be connected to any suitable display and/or recording device commonly in use for potentiometric measurements. The function of the differential amplifier is to subtract the common mode signal from each of the reference electrode and sodium electrode signals and to produce a potentiometric signal corresponding to the difference between the sodium-selective electrode and the reference electrode. The output of the amplifier is proportional to sodium activity which is proportional to sodium ion concentration.

The method of the invention will be largely understood from the foregoing description of the apparatus or system of FIG. 1. If the system is to be shut down for a period of time, say overnight, the ion-sensitive cannula 38 of the sodium electrode may be conditioned for the next use by leaving the cannula 38 exposed to a buffered solution of the type used in analysis. Such buffered solution may include a blood serum or plasma standard. Such buffered solution would provide human blood protein in contact with the sodium glass cannula 38 to speed the response of the ion-selective electrode when the system is again put in use and which exposure to a protein also avoids drift when the system is next put in use. As indicated heretofore, such conditioning with blood protein is contrary to known prior teachings. The sodium ion-selective glass of the cannula 38 may be left exposed to the buffered serum or plasma by avoiding, in any well known manner, drainage of such buffered solution from the cannual 38 and from the system when the system is at rest.

Alternatively, prior to running a first series of test samples, the sodium ion-selective glass may be conditioned by operating the system with a few specimens of plasma or sera standards. Conditioning by this alternative technique may be faster and more satisfactory than conditioning by the technique rst mentioned. In either event, the ion-selective sodium electrode is conditioned by exposure to blood protein and the ions which will affect the electrode in analysis, as well as the buffer solution.

As previously indicated, response of the ion-selective electrode is speeded by exposure of this electrode to heat above body temperature but not to a degree high enough to damage the sodium ion-selective electrode. While the heating of the sample has been illustrated and described with reference to a heating bath separate and apart from the combined electrode structure 30, it will be obvious to those versed in the art that a heated coil for the sample may be built into the electrode structure if desired. It will also be evident that instead of using a liquid filling solution in the chamber 41 of the ion-selective electrode, the electrolyte may be a solid, if desired, which encircles the cannual 38 in closecontacttherewith, which has an electrical Contact with the silver-silver chloride wire 44.

The aforementioned segmentation of the sample stream by immiscible uid segments such as gas is important to the method of analysis in that such immiscible fluid segments effectively tend to prevent the merging of one sample with another and the contamination of one sample by another, which blending of samples with each other at their extremities in the sample conduit would require longer exposure of each sample to the ion-selective electrode to produce an accurate potentiometric response for each sample. With a segmented sample stream exposed to the ion-selective electrode, the volume of the sample may be reduced and the time required for exposure of the sample to such electrode may be reduced to 18 seconds or less enabling a much faster sample rate of analysis than heretofore, provided that the immiscible fluid segments such as gas bubbles are also maintained in a small size range but naturally of a size sufficient to occlude the sample passageway. Polarizing of the ion-selective electrode is also effectively inhibited by maintaining such gas bubbles in the aforementioned small size range and reducing as far as is practical the time over which such a gas bubble is exposed t the ion-selective electrode. It is theorized that the thin liquid boundary of bubbles of such small size is sufficiently conductive to avoid polarization of the electrodes.

It has been brought out that the method of the invention efliectively avoids confusing initial transient responses of the Sodium-selective electrode to changes in concentrations between samples of potassium present in the samples. In recorder pen tracings of peak values of sodium in samples containing both sodium and potassium in different concentrations of potassium as well as different concentrations of sodium, no transient potassium effect has been evident. It is theorized that in the fast electrode response in accordance with the method of analysis previously discussed, the initial response of the sodium ion electrode to potassium is countered by the effect of the wash between samples of the aforementioned wash solution interposed in the segmented sample stream intermediate samples. As in the operation of the sampler disclosed by the aforementioned de Jong Patent 3,134,- 263, the wash solution is interposed in the segmented sample stream in segments of such solution, each of which Wash solution segments being interposed between a pair of gas segments.

While the presently preferred forms of the invention have been illustrated and described, it will be apparent to those versed in the art that the method of analysis may take other forms and is susceptible of various changes in details without departing from the principles of the invention.

What is claimed is:

1. A method of potentiometric analysis of a series of liquid samples for a constituent of interest utilizing a reference electrode and a sensing electrode having an ion-selective surface portion, said reference electrode and said ion-selection surface portion of said sensing electrode dening, in part, a ow-thru conduit, comprising the steps of:

flowing the liquid samples sequentially in a stream along said conduit;

introducing gas to segment the streamof samples to form gas segments intermediate each sample and its neighboring samples so that each sample comprises a liquid segment;

passing said flowing stream along said conduit to contact said reference electrode and said ion-selective surface portion of said sensing electrode with said sample liquid and gas segments in said flowing stream;

locating said electrodes with respect to each other such that each sample liquid segment in said stream bridges said reference electrode and said ion-selective surface portion of said sensing electrode; and measuring the electrical potential across said electrodes for analysis of said liquid sample segments.

2. The method as in claim 1, further including introducing into lsaid flowing stream additional gas segments in a buffered solution stream.

3. The method as in claim 1, wherein: said samples contain both sodium and potassium and said sensing electrode is a sodium-selective electrode, further including heating said stream in transit toward said electrodes so that the sample segments on exposure to the electrodes 'are at a tcmperautre of substantially 40-50 C., substantially eliminating a potassium transient effect on said sensing electrode.

4. The method as in claim 1, further including segmenting said sample stream exposed to said electrodes with segments of wash solution, the last-named segments being separated from sample segments by gas segments.

S. A method as defined in claim 1, wherein: said samples are blood samples, and further including:

conditioning of said sensing electrode prior to analysis by exposure of the latter over a period of time to a solution containing blood proteins and ions present in human blood to which the sensing electrode will respond, in amounts substantially the same as in human blood, and

immediately following such conditioning, owing a stream of such samples in said conduit.

6. The method as in claim 5, wherein: said conditioning includes flowing said solution past said electrodes for exposure of the latter thereto.

7- The method as dened in claim 5, wherein said conditioning of the sensing electrode is by immersion of the sensing electrode in said solution while said solution is still.

References Cited UNITED STATES PATENTS 3,134,263 5/1964 De .Tong 73--423 A 2,797,149 6/1957 Skeggs 23--253 R 3,151,052 9/1964 Arthur et al. 204-195 G 3,357,910 12/1967 Schiller 204-1 T 3,398,079 8/1968 Arthur et al 204-195 G OTHER REFERENCES Jacobson, Analytical Chemistry, vol. 38, No. 13, December 1966, pp. 1951-1954.

TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 

1. A METHOD OF POTENTIOMETRIC ANALYSIS OF A SERIES OF LIQUID SAMPLES FOR A CONSTITUENT OF INTEREST UTILIZING A REFERENCE ELECTRODE AND A SENSING ELECTRODE HAVING AN ION-SELECTIVE SURFACE PORTION, SAID REFERENCE ELECTRODE AND SAID ION-SELECTION SURFACE PORTION OF SAID SENSING ELECTRODE DEFINING, IN PART, A FLOW-THRU CONDUIT, COMPRISING THE STEPS OF: FLOWING THE LIQUID SAMPLES SEQUENTIALLY IN A STREAM ALONG SAID CONDUIT; INTRODUCING GAS TO SEGMENT THE STREAM OF SAMPLE TO FORM GAS SEGMENTS INTERMEDIATE EACH SAMPLE AND ITS NEIGHBORING SAMPLES SO THAT EACH SAMPLE COMPRISES A LIQUID SEGMENT; PASSING SAID FLOWING STREAM ALONG SAID CONDUIT TO CONTACT SAID REFERENCE ELECTRODE AND SAID ION-SELECTIVE SURFACE PORTION OF SAID SENSING ELECTRODE WITH SAID SAMPLE LIQUID AND GAS SEGMENTS IN SAID FLOWING STREAM; LOCATING SAID ELECTRODES WITH RESPECT TO EACH OTHER SUCH THAT EACH SAMPLE LIQUID SEGMENT IN SAID STREAM BRIDGES SAID REFERENCE ELECTRODE AND SAID ION-SELECTIVE SURFACE PORTION OF SAID SENSING ELECTRODE; AND MEASURING THE ELECTRICAL POTENTIAL ACROSS SAID ELECTRODES FOR ANALYSIS OF SAID LIQUID SAMPLE SEGMENTS. 